Recent evidence suggests that electrical coupling is a major mode of communication between neurons in the vertebrate retina. Further, the strength of this coupling appears highly dynamic. Converging evidence indicates that the dopaminergic synaptic pathways play a major role in modulating neuronal coupling under different states of dark and of light adaptation. However, the exact relationship between dopamine actions and the adaptational state of the retina is presently unclear. The purpose of this proposal is to examine changes in receptive field and tracer coupling size between horizontal cells and between All amacrine cells under changing conditions of retinal adaptation. These parameters will provide two, independent indexes of changes in neuronal coupling. These experiments will thus allow for direct examination of the modulation of neuronal circuitry caused by changing the ambient background illumination. The basic method will be intracellular recordings of cells in the superfused, isolated rabbit retina-eyecup. Receptive field size will be measured using a narrow slit of light displaced across the retinal surface. In the second phase of this study, selective dopaminergic D1 and D2 receptor agonists and antagonists will be applied to mimic or block the changes in coupling produced with light. These experiments will provide definitive data concerning the synaptic mechanisms underlying changes in neuronal coupling which occur with changes in the dark/light adaptational state of the retina. Understanding these mechanisms is crucial to develop in strategies for treatment of pathologies of retinal adaptation such as night blindness.